1. Field of the Invention
The present invention relates to a proton-conducting polymer electrolyte membrane which comprises polyazoles containing phosphonic acid groups and can, owing to its excellent chemical and thermal properties, be used for a variety of purposes and is particularly useful as polymer electrolyte membrane (PEM) in PEM fuel cells.
2. Description of Related Art
A fuel cell usually comprises an electrolyte and two electrodes separated by the electrolyte. In the case of a fuel cell, a fuel such as hydrogen gas or a methanol/water mixture is supplied to one of the two electrodes and an oxidant such as oxygen gas or air is supplied to the other electrode and chemical energy from the oxidation of the fuel is in this way converted directly into electric energy. The oxidation reaction forms protons and electrons.
The electrolyte is permeable to hydrogen ions, i.e. protons, but not to reactive fuels such as the hydrogen gas or methanol and the oxygen gas.
A fuel cell generally comprises a plurality of single cells known as membrane-electrode units (MEUs) which each comprise an electrolyte and two electrodes separated by the electrolyte.
Electrolytes employed for the fuel cell are solids such as polymer electrolyte membranes or liquids such as phosphoric acid. Recently, polymer electrolyte membranes have attracted attention as electrolytes for fuel cells. In principle, a distinction can be made between two categories of polymer membranes.
The first category encompasses cation-exchange membranes comprising a polymer framework containing covalently bound acid groups, preferably sulfonic acid groups. The sulfonic acid group is converted into an anion with release of a hydrogen ion and therefore conducts protons. The mobility of the proton and thus the proton conductivity is linked directly to the water content. Due to the very good miscibility of methanol and water, such cation-exchange membranes have a high methanol permeability and are therefore unsuitable for use in a direct methanol fuel cell. If the membrane dries, e.g. as a result of a high temperature, the conductivity of the membrane and consequently the power of the fuel cell decreases drastically. The operating temperatures of fuel cells containing such cation-exchange membranes are thus limited to the boiling point of water. Moistening of the membranes represents a great technical challenge for the use of polymer electrolyte membrane fuel cells (PEMFCs) in which conventional, sulfonated membranes such as Nafion are used.
Materials used for polymer electrolyte membranes are thus, for example, perfluorosulfonic acid polymers. The perfluorosulfonic acid polymer (e.g. Nafion) generally has a perfluorinated hydrocarbon skeleton such as a copolymer of tetrafluoroethylene and trifluorovinyl and a side chain bearing a sulfonic acid group, e.g. a side chain bearing a sulfonic acid group bound to a perfluoroalkylene group, bound thereto.
The cation-exchange membranes are preferably organic polymers having covalently bound acid groups, in particular sulfonic acid. Processes for the sulfonation of polymers are described in F. Kucera et al., Polymer Engineering and Science 1988, Vol. 38, No. 5, 783-792.
The most important types of cation-exchange membranes which have achieved commercial importance for use in fuel cells are listed below: The most important representative is the perfluorosulfonic acid polymer Nafion® (U.S. Pat. No. 3,692,569). This polymer can, as described in U.S. Pat. No. 4,453,991, be brought into solution and then used as ionomer. Cation-exchange membranes are also obtained by filling a porous support material with such an ionomer. As support material, preference is given to expanded Teflon (U.S. Pat. No. 5,635,041).
A further perfluorinated cation-exchange membrane can be produced as described in U.S. Pat. No. 5,422,411 by copolymerization of trifluorostyrene and sulfonyl-modified trifluorostyrene. Composite membranes comprising a porous support material, in particular expanded Teflon, filled with ionomers consisting of such sulfonyl-modified trifluorostyrene copolymers are described in U.S. Pat. No. 5,834,523. U.S. Pat. No. 6,110,616 describes copolymers of butadiene and styrene and their subsequent sulfonation to produce cation-exchange membranes for fuel cells.
A further class of partially fluorinated cation-exchange membranes can be produced by radiation grafting and subsequent sulfonation. Here, a grafting reaction, preferably using styrene, is carried out as described in EP667983 or DE19844645 on a previously irradiated polymer film. The side chains are then sulfonated in a subsequent sulfonation reaction. A crosslinking reaction can be carried out simultaneously with the grafting reaction and the mechanical properties can be altered in this way.
Apart from the above membranes, a further class of nonfluorinated membranes obtained by sulfonation of high-temperature-stable thermoplastics has been developed. Thus, membranes comprising sulfonated polyether ketones (DE 4219077, EP 96/01177), sulfonated polysulfone (J. Membr. Sci. 83 (1993) p. 211) or sulfonated polyphenylene sulfide (DE 19527435) are known. Ionomers prepared from sulfonated polyether ketones are described in WO 00/15691.
Further known membranes include acid-base blend membranes which are prepared as described in DE 19817374 or WO 01/18894 by mixing sulfonated polymers and basic polymers.
To improve the membrane properties further, a cation-exchange membrane known from the prior art can be mixed with a high-temperature-stable polymer. The production and properties of cation-exchange membranes comprising blends of sulfonated PEK and a) polysulfones (DE 4422158), b) aromatic polyamides (DE 42445264) or c) polybenzimidazole (DE 19851498) have been described.
However, a problem associated with such membranes is their complicated and thus expensive production, since it is usually necessary firstly to form different polymers which are subsequently cast, frequently with the aid of a solvent, to produce a film. To prepare the sulfonated polymers, it is usual to dissolve the PEK in a suitable solvent and subsequently react it with an aggressive sulfonating reagent, for example oleum or chlorosulfonic acid. This reaction is relatively critical, since the sulfonating reagent is a strong oxidant, so that degradation of the PEK cannot be ruled out. This would, in particular, have an adverse effect on the mechanical properties of the polymer. In a further process step, the sulfonated polymer is isolated and converted into the neutral form. The polymer then has to be brought back into solution. It is then possible, inter alia, to cast a polymer film from this solution. The solvent used for this purpose, for example N-dimethylacetamide, subsequently has to be removed. The process for producing such membranes is consequently complicated and thus expensive.
Uncontrolled sulfonation at many points on the polymer takes place in these sulfonation processes using very strong sulfonating agents. The sulfonation can also lead to chain rupture and thus to a worsening of the mechanical properties and finally to premature failure of the fuel cells.
Sulfonated polybenzimidazoles are also known from the literature. Thus, U.S. Pat. No. 4,634,530 describes a sulfonation of an undoped polybenzimidazole film with a sulfonating agent such as sulfuric acid or oleum in the temperature range up to 100° C.
Furthermore, Staiti et al., (P. Staiti in J. Membr. Sci. 188 (2001) 71), have described the preparation and properties of sulfonate polybenzimidazole. In this case, it was not possible to carry out the sulfonation of the polymer in the solution. Addition of the sulfonating agent to the PBI/DMAc solution results in precipitation of the polymer. To carry out the sulfonation, a PBI film was produced first and this was dipped into a dilute sulfuric acid. The samples were then treated at temperatures of about 475° C. for 2 minutes to effect sulfonation. The sulfonated PBI membranes have a maximum conductivity of only 7.5*10−5 S/cm at a temperature of 160° C. The maximum ion-exchange capacity is 0.12 meq/g. It was likewise shown that PBI membranes sulfonated in this way are not suitable for use in a fuel cell.
The production of sulfoalkylated PBI membranes by reaction of a hydroxyethyl-modified PBI with a sulfone is described in U.S. Pat. No. 4,997,892. On the basis of this technology, it is possible to produce sulfopropylated PBI membranes (Sanui et al., in Polym. Adv. Techn. 11 (2000) 544). The proton conductivity of such membranes is 10−3 S/cm and is thus too low for applications in fuel cells in which 0.1 S/cm is sought.
A disadvantage of all these cation-exchange membranes is the fact that the membrane has to be moistened, the operating temperature is limited to 100° C. and the membranes have a high methanol permeability. The reason for these disadvantages is the conductivity mechanism of the membrane, with the transport of the protons being coupled to the transport of the water molecule. This is referred to as the “vehicle mechanism” (K. -D. Kreuer, Chem. Mater. 1996, 8, 610-641).
A second category which has been developed encompasses polymer electrolyte membranes comprising complexes of basic polymers and strong acids. Thus, WO 96/13872 and the corresponding U.S. Pat. No. 5,525,436 describe a process for producing a proton-conducting polymer electrolyte membrane, in which a basic polymer such as polybenzimidazole is treated with a strong acid such as phosphoric acid, sulfuric acid, etc.
J. Electrochem. Soc., volume 142, No. 7, 1995, pp. L121-L123, describes doping of a polybenzimidazole in phosphoric acid.
In the case of the basic polymer membranes known from the prior art, the mineral acid (usually concentrated phosphoric acid) used for achieving the necessary proton conductivity is usually introduced after shaping of the polyazole film. The polymer here serves as support for the electrolyte consisting of the highly concentrated phosphoric acid. The polymer membrane in this case fulfills further important functions; in particular it has to have a high mechanical stability and serve as separator for the two fuel cells mentioned at the outset.
A significant advantage of such a membrane doped with phosphoric acid is the fact that a fuel cell in which such a polymer electrolyte membrane is used can be operated at temperatures above 100° C. without the moistening of the fuel cell which is otherwise necessary. This is due to the ability of the phosphoric acid to transport protons without additional water by means of the Grotthus mechanism (K. -D. Kreuer, Chem. Mater. 1996, 8, 610-641).
The possibility of operation at temperatures above 100° C. results in further advantages for the fuel cell system. Firstly, the sensitivity of the Pt catalyst to impurities in the gas, in particular CO, is greatly reduced. CO is formed as by-product in the reforming of the hydrogen-rich gas comprising carbon-containing compounds, e.g. natural gas, methanol or petroleum spirit, or as intermediate in the direct oxidation of methanol. The CO content of the fuel typically has to be less than 100 ppm at temperatures of <100° C. However, at temperatures in the range 150-200° C., 10,000 ppm or more of CO can also be tolerated (N. J. Bjerrum et al., Journal of Applied Electrochemistry, 2001, 31, 773-779). This leads to significant simplifications of the upstream reforming process and thus to cost reductions for the total fuel cell system.
A great advantage of fuel cells is the fact that the electrochemical reaction converts the energy of the fuel directly into electric energy and heat. Water is formed as reaction product at the cathode. Heat is thus generated as by-product in the electrochemical reaction. In the case of applications in which only the electric power is utilized for driving electric motors, e.g. in automobile applications, or as replacement for battery systems in many applications, the heat has to be removed in order to avoid overheating of the system. Additional, energy-consuming equipment is then necessary for cooling, and this further reduces the total electrical efficiency of the fuel cell. In the case of stationary applications such as central or decentralized generation of power and heat, the heat can be utilized efficiently by means of existing technologies, e.g. heat exchangers. High temperatures are sought here to increase the efficiency. If the operating temperature is above 100° C. and the temperature difference between ambient temperature and the operating temperature is large, it is possible to cool the fuel cell system more efficiently or employ small cooling areas and dispense with additional equipment compared to fuel cells which have to be operated at below 100° C. because of the moistening of the membrane.
However, besides these advantages, such a fuel cell system also has disadvantages. Thus, the durability of membranes doped with phosphoric acid is still in need of improvement. Here, the life is, in particular, significantly reduced by operation of the fuel cell below 100° C., for example at 80° C. However, it has to be noted in this context that the cell has to be operated at these temperatures during start-up and shutdown of the fuel cell.
The previously known acid-doped polymer membranes based on polyazoles display a favorable property profile. However, owing to the applications desired for PEM fuel cells, in particular in the automobile sector and in decentralized power and heat generation (stationary sector), these still need to be improved overall. Thus, the production of membranes doped with phosphoric acid is relatively expensive, since it is usual firstly to form a polymer which is subsequently cast with the aid of a solvent to produce a film. After the film has been dried, it is doped with an acid in a final step. The previously known polymer membranes therefore have a high content of dimethylacetamide (DMAc) which cannot be removed completely by means of known drying methods.
Furthermore, the performance, for example the conductivity, of known membranes is still in need of improvement.